One technology for after-treatment of engine exhaust utilizes three-way catalyst (TWC) devices, which facilitate certain chemical reactions to occur between NOx in the exhaust and ammonia (NH3). The TWC facilitates the reaction between NH3 and NOx to convert NOx into nitrogen (N2) and water (H2O). However, as recognized by the inventors herein, issues with reactivity arise when oxygen concentrations at the TWC are too high. For example, the NH3 may react with oxygen to produce nitrous oxide instead of reducing the NOx constituents. This may force engine operating parameters to avoid operating under lean air/fuel ratios, thereby decreasing a fuel efficiency. In some examples, additionally or alternatively, NOx may entropically decompose in the presence of a catalyst to form N2 and O2. However, this decomposition may also be hampered by excess O2 being present at the catalyst. In this way, a method for reducing a concentration of O2 at the catalyst is desired.
Other attempts to address NOx reduction include flow an oxygen containing gas into an oxygen separator and/or oxygen filter. One example approach is shown by Jankowiak et al. in U.S. Pat. No. 8,852,409. Therein, an electric oxygen separator receives oxygen containing gas and separates the oxygen from the remainder of the gas. The separator is further adapted to expel the oxygen to an ambient atmosphere or to a receiver tank where the oxygen is stored. The separator includes one or more check valves and manifolds to direct the flow of oxygen through the system.
However, the inventors herein have recognized potential issues with such systems. As one example, the oxygen separator utilizes electricity to separate oxygen from the oxygen containing gas. This reduces fuel efficiency benefits realized from extending a lean operation of the engine. Additionally, including one or more valves and manifolds increases a manufacturing cost and introduces additional components susceptible to degradation. Furthermore, the valves and manifolds increase a packaging constraint of an exhaust passage, decreasing fuel economy.
In one example, the issues described above may be addressed by a method for enriching oxygen in an exhaust gas stream in a second exhaust passage having an oxygen filter, flowing oxygen poor exhaust gas to a first exhaust passage comprising a catalyst, and merging the oxygen rich and oxygen poor exhaust gas streams at an intersection downstream of the oxygen filter and the catalyst. In this way, a TWC in the first exhaust passage receives oxygen poor exhaust gas to increase NOx reduction efficiency and extend lean engine operation.
As one example, a main exhaust passage comprises a first catalyst upstream of a bifurcation of the main exhaust passage. In one example, the first catalyst uninterruptedly receives exhaust gas exhausted from an engine. After flowing through the first catalyst, the exhaust gas flows to the bifurcation, where the exhaust gas may flow into the first exhaust passage or the second exhaust passage. Exhaust gas flowing to the second passage flows through the oxygen filter, resulting in an oxygen enriched (oxygen rich) exhaust flow through the second passage. As such, an oxygen deficient (oxygen poor) exhaust flow remains upstream of the oxygen filter. In this way, exhaust gas flowing into the first passage merges with the oxygen poor exhaust flow, where the mixture flows to a second catalyst. By doing this, the mixture comprises a lower concentration of oxygen than exhaust gas flowing to the first catalyst from the engine. As such, a NOx reduction efficiency is greater in the second catalyst than the first catalyst. In this way, lean engine operation may be extended. In one example, lean engine operation is not terminated in response to the NOx reduction efficiency decreasing below a threshold efficiency (e.g., less than an emission standard) due to the configuration of the first and second exhaust passages.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.